Liquid crystal display devices adopting nematic liquid crystal display elements, which have conventionally been widely used as numeric-value-segment-type display apparatuses such as watches and calculators, have recently been also used in word processors, note-type personal computers, car-use liquid crystal televisions, and other apparatuses.
Generally, a liquid crystal display element is provided with transparent substrates on which electrode lines for turning on or turning off pixels are formed. For example, in an active-matrix-type liquid crystal display device, active elements such as thin film transistors are provided, on the substrates together with the electrode lines, as switching means for selectively driving pixel electrodes which apply a voltage to liquid crystal. Also, in a liquid crystal display device which carries out color displaying, color filter layers of red, green, and blue are provided on the substrates.
The liquid crystal display element such as above adopts a liquid crystal displaying system which is suitably selected in accordance with the twist angle of the liquid crystal. For example, the active-driving-type twisted nematic liquid crystal displaying system (hereinafter, referred to as the TN system) and the multiplex-driving-type super-twisted nematic liquid crystal display system (hereinafter, referred to as the STN system) are widely known.
In the TN system, displaying is carried out by aligning nematic liquid crystal molecules in a 90.degree. twist, and a ray of light is directed along the twisted direction. In the STN system, displaying is carried out by taking an advantage of the fact that the transmittance suddenly changes in a vicinity of the threshold value of a voltage applied to the liquid crystal when the twist angle of the nematic liquid crystal molecules is made larger than 90.degree..
Since the STN system utilizes the birefringence effect of the liquid crystal, on the background of the display screen, a distinct color is generated due to interference of colors. In order to overcome such a problem so as to realize black-and-white displaying in the STN system, it is effective to use an optical compensation plate. A displaying system adopting the optical compensation plate can be roughly classified into two displaying systems: (1) The double layered super-twisted nematic phase compensation system (hereinafter, referred to as the DSTN system) and (2) the film-type phase compensation system (hereinafter, referred to as the film-addition-type system) in which a film having optical anisotropy is provided.
In the DSTN system, a double-layered structure is adopted wherein (1) a displaying-use liquid crystal cell and (2) a liquid crystal cell which is twist-aligned by a twist angle in a direction opposite to that of the displaying-use liquid crystal cell are provided. In contrast, the film-addition-type system adopts an arrangement wherein a film having optical anisotropy is provided. Of the two systems, the film-addition-type system is considered to be more prospective from the standpoint of light weight and low costs. Since the application of such phase compensation systems makes it possible to improve the black-and-white display characteristics, color STN liquid crystal display devices capable of color displaying have been realized by means of providing a display device of the STN system with color-filter layers.
The TN system can be roughly classified into (1) a normally-black system and (2) a normally-white system. In the normally-black system, a pair of polarization plates are positioned so that the respective polarization axes thereof are parallel to each other so as to display black during a state in which no voltage is applied to the liquid crystal layer (off state). In the normally-white system, a pair of polarization plates are positioned so that the respective polarization axes thereof are orthogonal to each other so as to display white during the off state. The normally-white system is considered to be more prospective from the standpoint of a display contrast, color reproducibility, and viewing-angle-dependency of the display, etc.
Incidentally, the described TN liquid crystal display device has a problem of increased viewing-angle-dependency in that the contrast of a displayed image is changed depending on the direction in which and the angle by which the displayed image is observed by the observer, due to the fact that (1) the liquid crystal molecules exhibit refractive index anisotropy .DELTA.n and (2) the alignment direction of the liquid crystal molecules is tilted with respect to the substrates.
FIG. 12 schematically shows a cross sectional arrangement of a TN liquid crystal display element 31. The arrangement of FIG. 12 is induced by the application of a voltage for half-tone displaying so that a liquid crystal molecule 32 slants upward slightly. In the liquid crystal display element 31, (1) linearly polarized light 35 passing through the TN liquid crystal display element 31 in a direction normal to the surfaces of substrates 33 and 34 and (2) linearly polarized light 36 and 37 respectively passing through the TN liquid crystal display element 31 in directions inclined with respect to the normal direction cross the liquid crystal molecule 32 at different angles. Thus, since the liquid crystal molecule 32 exhibits refractive index anisotropy .DELTA.n, when the linearly polarized light 35, 36, and 37 transmit through the liquid crystal molecule 32 in the respective directions, ordinary light and extraordinary light are generated. As a result, the linearly polarized light 35, 36 and 37 are respectively converted into elliptically polarized light according to a phase difference between the ordinary light and the extraordinary light. This is the cause of the viewing-angle-dependency.
Further, in an actual liquid crystal layer, the liquid crystal molecule 32 has different tilt-angles (a) in a vicinity of a midway portion of the substrate 33 and the substrate 34 and (b) in respective vicinities of the substrate 33 and the substrate 34. Also, the liquid crystal molecule 32 is twisted by 90.degree. about the axis (normal direction).
As described, the linearly polarized light 35, 36, and 37 passing through the liquid crystal layer are subjected to various birefringence effects depending on the direction or the angle of the travel. This results in complex viewing-angle-dependency.
Specifically, as such viewing-angle-dependency, the following phenomena are observed. When the viewing direction is inclined towards the standard viewing direction, i.e., the downward direction of the display surface, from the direction normal to the display screen, above a certain angle, (1) coloring of the displayed image is observed (hereinafter referred to as coloration phenomenon) and (2) the black and white of the displayed image is reversed (hereinafter referred to as reversion phenomenon). Also, when the viewing angle is inclined towards the opposite viewing direction, i.e., the upward direction of the display screen, sudden lowering of the contrast is observed.
Further, the described liquid crystal display device has a drawback in that the viewing angle becomes smaller with an increase in size of the display screen. When a large liquid crystal display screen is viewed from the front with a close distance, there is a case where different colors are observed in a displayed image on the upper portion and the lower portion of the display screen due to the effect of the viewing-angle-dependency. This is caused by a wider range of viewing angle required to encompass the entire screen surface, which is equivalent of a viewing direction increasingly far off-center.
In order to improve such viewing-angle-dependency, for example, Japanese Laid-Open Patent Applications No. 600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest an arrangement wherein an optical phase difference plate (phase difference film) having optical anisotropy is inserted as an optical element between the liquid crystal display element and one of the polarization plates.
In this arrangement, the light, having converted to elliptically polarized light from linearly polarized light in the course of its passage through the liquid crystal molecules having refractive index anisotropy, is allowed to pass through the optical phase difference plate which is provided on a side or on the both sides of the liquid crystal layer having refractive index anisotropy. This ensures that the change in the phase difference of the ordinary light and the extraordinary light with viewing angles is compensated so that the elliptically polarized light is converted again back to the linearly polarized light, thereby permitting the viewing-angle-dependency to be improved.
As such an optical phase difference plate, for example, Japanese Laid-Open Patent Application No. 313159/1993 (Tokukaihei 5-313159) discloses an optical phase difference plate wherein one of the principal refractive index directions of a refractive index ellipsoid is made parallel to the direction normal to the surface of the optical phase difference plate. However, even with this optical phase difference plate, there is a limit in suppressing the reversion phenomenon in the standard viewing direction.
In order to overcome this limit, Japanese Laid-Open Patent Application No. 75116/1994 (Tokukaihei 6-75116) suggest an optical phase difference plate having an arrangement wherein the principal refractive index direction of the refractive index ellipsoid is inclined with respect to the direction normal to the surface of the optical phase difference plate. As an optical phase difference plate having this arrangement, two types of optical phase difference plates 1 and 2 are suggested.
1 In this optical phase difference plate, of three principal refractive indices of the refractive index ellipsoid, the direction of the smallest refractive index is made parallel to the surface of the optical phase difference plate, and the direction of one of the remaining refractive indices is inclined by an angle .theta. with respect to the surface of the optical phase difference plate, and the direction of the other remaining refractive index is also inclined by the angle .theta. with respect to a direction normal to the optical phase difference plate, wherein the value of .theta. satisfies the condition 20.degree..ltoreq..theta..ltoreq.70.degree.. PA1 2 In this optical phase difference plate, three principal refractive indices n.sub.a, n.sub.b, and n.sub.c of the refractive index ellipsoid are related to each other by the relation n.sub.a =n.sub.c &gt;n.sub.b, and (1) the direction of the principal refractive index n.sub.b parallel to the direction normal to the surface of the optical phase difference plate and (2) the direction of the principal refractive index n.sub.a or n.sub.c on the surface of the optical phase difference plate are inclined in a clockwise direction or in a counterclockwise direction about the direction of the principal refractive index n.sub.a or n.sub.c on the surface of the optical phase difference plate. Namely, the refractive index ellipsoid is inclined with respect to the optical phase difference plate.
In the two types of the optical phase difference plates 1 and 2, the former can adopt an optical phase difference plate of either a uniaxial type or a biaxial type. On the other hand, the latter can adopt an arrangement wherein two optical phase difference plates are provided in a pair instead singly, and the inclined direction of the principal refractive index n.sub.b of each of the pair of optical phase difference plates is set to 90.degree..
In a liquid crystal display device having an arrangement wherein at least one such an optical phase difference plate is provided between the liquid crystal display element and the polarization plates, it is possible to suppress to some degree the change in contrast, the coloration phenomenon, and the reversion phenomenon which are generated depending on the viewing angle of the display screen.
Also, techniques for eliminating the reversion phenomenon have been suggested. For example, Japanese Laid-Open Patent Applications No. 186735/1982 (Tokukaisho 57-186735) discloses a so-called pixel dividing method in which display patterns (pixels) are sorted into a plurality of regions, and each region is independently subjected to alignment control so as to give a distinguishable viewing angle characteristic to each region. In this method, the liquid crystal molecules slant upward in different directions in each region, thereby eliminating the viewing-angle-dependency.
Also, Japanese Laid-Open Patent Applications No. 118406/1994 (Tokukaihei 6-118406) and Japanese Laid-Open Patent Applications No. 194645/1994 (Tokukaihei 6-194645) respectively disclose a technique in which the pixel dividing method is used in conjunction with optical phase difference plates.
In a liquid crystal display device disclosed in Japanese Laid-Open Patent Applications No. 118406/1994 (Tokukaihei 6-118406), in order to improve the contrast, optically anisotropic films (optical phase difference plates) are provided between the liquid crystal panel and the polarization plates. The compensation plates (optical phase difference plates) disclosed in Japanese Laid-Open Patent Applications No. 194645/1994 (Tokukaihei 6-194645) have substantially no surface refractive index being parallel to the surfaces of the compensation plates, and the refractive index in a direction perpendicular to the surfaces of the compensation plates is set so as to be smaller than the refractive index in the surfaces, and therefore has a negative refractive index. Thus, when a voltage is applied, positive refractive index generated in the liquid crystal display element is compensated so that the viewing-angle-dependency is lowered.
However, due to a demand for a liquid crystal display device having a wider viewing angle and a higher displaying quality in today's market, a further advancement in improving the viewing-angle-dependency is demanded. The optical phase difference plates disclosed in the above-mentioned U.S. Patents are not sufficient for meeting such a demand, and therefore there is a room for further improvements.
Also, in the pixel dividing method for eliminating the reversion phenomenon, the reversion phenomenon and the viewing-angle-dependency are eliminated because the viewing angle characteristics become substantially symmetrical when viewed from upward and downward directions. However, the pixel dividing method still has a problem in that the contrast is lowered when viewed from upward and downward directions. Thus, the displayed black is paled, and is perceived as grey. Also, the described prior art adopting the pixel dividing method has a drawback in that the viewing-angle-dependency is generated when viewed from left and right directions.
Also, in the pixel dividing method used in conjunction with the optical phase difference plates, when the viewing direction is inclined, the coloration phenomenon is generated at a 45.degree. incline. Further, because a liquid crystal element in which the pixels are sorted into regions at the same ratio is adopted, there is a limit in suppressing the lowering of contrast generated when viewed from the upward and the downward directions, for the following reason.
In the pixel dividing method, since the dividing ratio of the pixels is the same, the viewing angle characteristics of the liquid crystal display element in the standard viewing direction (direction in which the contrast improves from a direction perpendicular to the screen) and the opposite viewing direction (direction in which the contrast lowers from a direction perpendicular to the screen) are averaged. However, in practice, because the viewing angle characteristics in the standard viewing direction and the opposite viewing direction are inversely related to each other, even when the pixel dividing method is used in conjunction with the optical phase difference plates, it is difficult to uniformly suppress the lowering of contrast in the upward and downward directions. Especially, when the viewing direction is inclined towards the standard viewing direction, the reversion phenomenon and darkening of display image are generated.